1. Field of the Invention
The present invention relates to polymeric microgel and to its method of preparation. More particularly, this invention relates to the preparation of polymeric microgel by aqueous emulsion polymerization techniques.
2. Brief Description of the Prior Art
Microgel is a polymeric particle of colloidal dimensions, for example, about 0.001 to 10 microns in diameter, which is gelled or crosslinked. The microgel is prepared by dispersion polymerization techniques in which the crosslinked polymer particles are comparable in size to non-crosslinked polymer particles prepared by dispersion polymerization techniques. Strong solvents may cause the microgel to swell but they will not dissolve the particles. Microgels have been known for some time and recently much work has been done involving their use in coating compositions.
The microgels significantly affect and modify the rheological properties of the coating compositions and/or the physical properties of the coatings into which they are incorporated. When formulated into coatings, microgel improves the spraying efficiency of the coatings enabling complete coverage in fewer passes of the spray gun. Microgel also improves the sag resistance of the coating and the metallic pigment pattern control of coating compositions using metallic pigments such as aluminum flake. Typical patents describing microgel and its method of preparation are as follows:
British Pat. No. 967,051 and U.S. Pat. No. 4,290,932 disclose a process for preparing a polymeric microgel via aqueous emulsion polymerization techniques. The polymer is prepared by polymerizing a mixture of vinyl monomers, at least one of which contains at least two ethylenically unsaturated double bonds, via aqueous emulsion polymerization techniques to form the resulting crosslinked polymeric microgel.
U.S. Pat. No. 4,296,013 discloses vinylidene chloride polymer microgels. The microgels are obtained by emulsion polymerizing vinylidene chloride, a copolymerizable ethylenically unsaturated monomer such as ethyl methacrylate, and about 0.1 to 10 parts by weight of a copolymerizable crosslinking polyfunctional comonomer such as 1,4-butanediol diacrylate.
U.S. Pat. No. 4,209,435 discloses a process for preparing a stable aqueous dispersion of polymer particles. The method comprises the steps of forming a sterically stabilized dispersion in a non-aqueous liquid of a first polymer which may be crosslinked and is insoluble in the non-aqueous liquid and in water; polymerizing monomer in the same non-aqueous liquid in the presence of particles of the first polymer and of a steric stabilizer so as to form a second polymer which is soluble in the aqueous medium; and transferring the resultant polymer particles from the non-aqueous medium to the aqueous medium.
U.S. Pat. No. 4,322,328 discloses sterically stabilized aqueous polymer dispersions of at least 20 percent solids content are made by free radical initiated polymerization of monomers in an aqueous medium at a temperature at least 10.degree. C. higher than the glass transition temperature of the polymer to be formed, in the presence of a compound which is soluble in the aqueous medium and which contains in the molecule a polymer component of molecular weight of at least 1000 which is solvatable by the aqueous medium and an unsaturated group which can copolymerize with the monomers.
U.S. Pat. No. 4,324,714 discloses coating compositions containing crosslinked vinylidene chloride polymer microgel powders which are prepared in accordance with U.S. Pat. No. 4,296,013 mentioned above.